Copolyimides

ABSTRACT

Copolyimide foams made from lower alkyl esters of tetracarboxylic acids and two or more diamines. Precursors for such copolyimides. Processes for making the precursors and for converting them to copolyimide foams.

This application is a division of application Ser. No. 522,765 filedNov. 11, 1974 (now U.S. Pat. No. 3,966,652).

The present invention relates to copolyimides and, more specifically, tonovel, improved copolyimide foams which, in addition to their otherattributes, are structurally stable and remain flexible and resilientover a wider temperature range than those heretofore available. In otheraspects our invention relates to novel precursors for such copolyimidefoams and to novel methods for making and foaming the precursors.

U.S. Pat. Nos. 3,726,834 and 3,793,281 issued Apr. 10, 1973, and Feb.19, 1974, to Acle; U.S. Pat. No. 3,554,935 issued Jan. 12, 1971, toKnapp et al; and U.S. Pat. No. 3,554,939 issued the same day to Lavin etal disclose copolyimide foams made from tetracarboxylic acid derivativesand various diamines. One disadvantage of the polyimides disclosed inthese patents is that they become brittle and structurally unstable atcryogenic temperatures (-300° F. or lower). If struck or otherwisesubjected to mechanical stress at such temperatures, the cellularstructure of these foams collapses; and they may even crumble into apowder.

We have unexpectedly discovered that this disadvantage of the patentedpolymers can be overcome and foams which are structurally stable atelevated temperatures but remain flexible and resilient at cryogenictemperatures obtained by substituting a novel mixture of diamines whichproduces longitudinal disorder in the polymer chain for the diaminessuggested by the listed patents. This also permits foaming to be carriedout at lower temperatures than has ordinarily been possible heretoforewhich is important for obvious reasons.

Their unique properties make the novel copolyimide foams we haveinvented useful in a variety of applications involving a wide range oftemperatures. They may, for example, be used to particular advantage asheat shields in temperatures as high as 600+° F. and as gaskets inapparatus operating at or subjected to cryogenic temperatures.

Another attribute of our novel foams, which is important in a variety ofapplications, is that they give off essentially no smoke when heated todegradation temperatures. On the order of ten percent of the degradationproducts of typical prior art polyimides, in contrast, come off assmoke.

The initial step in making our novel cryogenic foams involves thepreparation of a resinoid or solid state solution type of precursor.This precursor preferably contains essentially equimolar proportions ofone or more, chemically pure, lower alkyl diesters of a tetracarboxylicacid and a mixture of two or more diamines. At least one of the diaminesmust be a heterocylic diamine, and any diamine which is not heterocyclicis preferably an aromatic meta or para-substituted diamine whichcontains no aliphatic moieties.

If diamines which do have aliphatic moieties are used, the resultingfoam will be significantly less structurally stable. Also, smoke will beproduced if the foam is heated above its thermal degradationtemperature. Thus diamine mixtures such as those disclosed in U.S. Pat.No. 3,424,718 issued Jan. 28, 1969, to Angelo would be decidedlyinferior for our purposes to the diamine mixtures we preferably employ.

Mixtures of diamines must be employed so that there will be a randomdistribution of dissimilar, recurring, aromatic and heterocyclic unitsin the polymer chain. As will be shown hereinafter, the advantages ofour novel foams are not possessed when one or a mixture of aromaticdiamines is employed; and precursors like ours but containing only aheterocyclic, diamine cannot be foamed at all.

Examples of suitable aromatic diamines are 3,3'-diamino diphenylsulfone; 4,4'-diamino diphenyl sulfone; 4,4'-diamino diphenyl sulfide;3,3'-diamino diphenyl ether; 4,4'-diamino diphenyl ether; benzidine,meta-phenylene diamine; and paraphenylene diamine.

The heterocyclic diamines we can employ include 2,6-diamino pyridine and3,5-diamino pyridine.

We can use from 95 parts by weight of heterocylic diamine to 5 parts ofaromatic diamine to 95 parts of aromatic diamine to 5 parts ofheterocyclic diamine.

The above-cited Acle patents do of course indicate that heterocyclicdiamines may be used in the polyimides therein disclosed. However, Acledoes not attribute any special significance to the use of such diamines,let alone recognize that the presence of a heterocyclic diamine and thecombination therewith of an aromatic diamine are prerequisites forresiliency, flexibility, and structural stability at cryogenictemperatures.

Another U.S. patent which discloses polyimides made from heterocyclicdiamines is U.S. Pat. No. 3,661,849 issued May 9, 1972, to Culbertson.However, there is no suggestion in this patent that the polymers thereindisclosed can be obtained in foamed form, that a mixture of diamines canbe used, or that the polymers are usable at cryogenic temperatures.Furthermore, Culbertson's diamines are quite different from those weuse.

The ester component of the precursor is prepared by reacting thetetracarboxylic acid or its anhydride with one or more aliphaticalcohols at up to reflux temperature to esterify it.

The acidic constituent we preferably employ is3,3'4,4'-benzophenonetetracarboxylic acid dianhydride. The esters can bemade from the acid instead of the dianhydride, if desired. However, asthe anhydride is the commercially available and most stable form, itwill typically be employed.

The alcohols we use are those containing from 1 to 3 carbon atoms. Ethylalcohol free of denaturants and additives is in many cases preferred.

Mixtures of C₁ -C₃ esters can also be employed to advantage. Theseafford additional control over the foaming of the precursor as volatilescome off at different temperatures if mixed esters are used.

The mixture present after esterification is complete will typically becooled before the diamines are added strictly as a safety measure. Thediamines are then added and the mixture stirred, typically at reflux,until the diamines are dissolved.

Excess alcohol is removed from the resulting product at reduced pressureuntil it becomes a thick syrup.

Removal of the solvent is then interrupted, and a surfactant ispreferably added to control the pore size and the cellular structure ofthe foam which will ultimately be made. We employ from 0.1 to 10 partsby weight of surfactant for each 100 parts of resin constituent for thispurpose.

One suitable surfactant is Union Carbide L-5430 silicone surfactant.That company's L-5410 and L-530 surfactants are also suitable as arevarious silicone surfactants available from Dow Chemical and GeneralElectric.

The surfactant is stirred into the syrupy mixture at a temperature offrom 77° to 131° F. Excess solvent is then removed by heating themixture at 150° to 160° F. under reduced pressure for 6-8 hours.

At the end of this cycle the product is a fragile resinoid which ispulverized into a powder with a maximum particle size of 40 mesh.

This powdered resinoid is further processed to reduce its volatilescontent to a value which is one to five percent lower than thetheoretical volatiles content of the resinoid (The theoretical volatilescontent is the weight percent of alcohol and water in thebenzophenonetetracarboxylic ester and diamine molecules released duringthe condensation polymerization reactions. It is these volatiles thatfoam the resinoid during the foaming process). The volatiles content ofthe resin is lowered below the theoretical value to avoid the presenceof free solvent as this would interfere with the foaming process. If thevolatiles content is not reduced prior to foaming, volatilization willoccur too fast in the foaming step, causing large voids in the foams andirregular cell structure.

The volatiles content of the resinoid precursor can be reduced to thedesired level by heating it at a temperature of from 150° to 250° F. fora period of five minutes to 24 hours. After this step the resinoid ispulverized again to particles not exceeding 40 mesh in size.

The resulting resinoid powder has a long shelf life; it is free-flowingeven after being stored for several months.

This resinoid precursor can be foamed by the free rise (or unrestricted)method or in a mold. If the resin is foamed in a mold, pressures of from1 to 5 psi developed by the gases released during the foaming processcan be utilized to compress the foam and increase its density. Inaddition, external pressure can be applied to obtain a still higherdensity, if desired. In most cases the total pressure applied to themold should not exceed 50 psi.

The resinoids are foamed by the application of heat and preferably atrelatively low temperatures of 220° to 325° F. This produces the mosthomogeneous distribution of and the smallest cells.

The foam can be heated to a temperature in the range of 550°-600° F. tocomplete the cure of the polymer, if desired.

No solvents or catalysts or other reactants are used in the foamingstep, which makes our novel process decidedly superior to heretoforeproposed processes for making polyimide foams such as those described inU.S. Pat. No. 3,249,561 issued May 3, 1966, to Hendrix; U.S. Pat. No.3,282,897 issued Nov. 1, 1966, to Angelo; U.S. Pat. No. 3,575,936 issuedApr. 20, 1971, to Dinan; U.S. Pat. No. 3,310,506 issued Mar. 21, 1967,to Ambroski et al; U.S. Pat. No. 3,520,837 issued July 21, 1970, toWilson; and U.S. Pat. No. 3,542,703 issued Nov. 24, 1970, to DeBrunner.

From the foregoing it will be apparent to the reader that one primaryand important object of our invention resides in the provision of novel,improved polyimide foams which remain flexible, resilient, andstructurally stable over a wider range of service temperatures than hasheretofore been the case.

Another primary object of our invention is the provision of novel,improved polyimide foams which remain flexible, resilient, andstructurally stable at cryogenic temperatures.

A further primary object of our invention resides in the provision ofnovel, improved polyimide foams in accord with the preceding objectswhich will give off essentially no smoke when heated to degradationtemperature.

Other primary objects of the invention reside in the provision of novelprecursors for polyimide foams having the advantages identified in thepreceding paragraphs and in the provision of novel methods for preparingthe precursors and for converting them to foams.

Further, related, and also primary objects of the invention reside inthe provision of precursors which have a long storage life and in theprovision of methods for converting the precursors to foams which do notrequire high temperatures or pressures, solvents, catalysts or otherundesirable steps or chemicals and which are capable of producing foamswith small and homogeneously distributed cells.

Other objects and advantages and additional novel features of ourinvention will be apparent to those skilled in the relevant arts fromthe foregoing general description of the invention, from the appendedclaims, and from the following examples, which are intended only toillustrate and not restrict the scope of the invention.

EXAMPLE I

A precursor of the character we contemplate was prepared generallyfollowing the method disclosed in U.S. Pat. No. 3,506,583 issued Apr. 4,1970, to Boran et al.

Specifically, 3,3'4,4'-benzophenonetetracarboxylic acid dianhydride(322.2g, 1.0M) was dissolved in 1,000 mls of reagent grade ethanol andrefluxed with stirring for 60 minutes to convert it to the diethylester. The mixture was cooled to 131° F., and 54.6 g (0.5 M) of2,6-diamino pyridine and 124.15 g (0.5 M) of 4,4'-diamino diphenylsulfone were added.

The mixture was refluxed at 174.2° F. until complete solution occurred.Excess ethanol was removed under reduced pressure at a maximumtemperature of 140° F. until the mixture became a thick syrup.

A surfactant (11.9 g of Union Carbide L-5430) was added and excesssolvent removed by heating the mixture in a vacuum oven for 8 hours at140° F. This resulted in a brittle residue which was pulverized,producing a free-flowing resinoid powder with a 40 mesh maximum particlesize.

The powdered resinoid was placed in a flat, shallow dish and heated inan air circulating oven at 200° F. for a period of 6 hours to reduce itsvolatiles content. Thereafter, the resinoid was again pulverized into apowder with a maximum particle size of 40 mesh.

The melting point of this resinoid was 200°-225° F.; and its volatilescontent fell within the range of 1 to 5 percent below the theoreticalvalue, indicating incipient polymerization; i.e., that the resinoidcontained a small amount of polymerized material in addition tounreacted ester and diamines.

The resin was free-flowing and remained so after having been stored forseveral months.

The time and temperature employed in the step of heating the resinoid toreduce its theoretical volatiles content are not critical and can bevaried, if desired. For example, the resinoid can be heated at thetemperature employed in Example I for 3 to 6 hours; at a temperature of250° F. for 15 to 60 minutes; or at a temperature of 150°-160° F. for 16to 24 hours.

EXAMPLE II

This example illustrates how the resinoids we have invented can befoamed by the free-rise or unrestricted method.

A few grams of a resinoid prepared as described in Example I was placedon a sheet of aluminum foil and transferred to an air-circulating ovenheated to 250° F. The temperature was maintained at 250° F. until theresin melted (15 minutes).

The temperature in the oven was then raised to 320° F. at a rate of 1°to 2° F. every 10 minutes. Thereafter, the temperature was raised to550° F. over a period of 30 minutes. The material was kept at thistemperature for 60 minutes to complete the cure of the resin.

The product was a three-inch diameter bun of light yellow, resilientfoam with a homogeneous cell structure and a density of two pounds percubic foot.

Again, there is nothing critical in the particular operating parametersdescribed in the example. An intermediate oven temperature as low as220° and as high as 325° F. can be employed, and this range can beextended if maximum homogenity and minimum cell size are not required.The temperature could have been raised more rapidly to the 550° F. level(for example, in 20 minutes). The curing of the resin may be completedin as few as 30 minutes at temperatures ranging up to 600° F. with thecuring time ordinarily being decreased as the curing temperature isincreased.

EXAMPLE III

This example describes a test conducted to show that the novel polyimidefoams we have invented remain stable and resilient at cryogenictemperatures.

A copolyimide artifact obtained by foaming the Example I precursor bythe Example II process was immersed in liquid argon (approximately -300°F.) for several minutes. The foam was removed from the liquid argon andimmediately crushed with sharp blows. The foam remained resilient, andits cellular structure was not affected by the cryogenic temperature orby the destructive test to which the foam was subjected.

EXAMPLE IV

To show that related, heretofore proposed polyimide foams which arehomopolymers (see, for example U.S. Pat. Nos. 3,483,144 issued Dec. 9,1969, to Lavin et al and U.S. Pat. No. 3,575,981 issued Apr. 20, 1971,to Le Blanc et al) do not have the novel properties which ours possess,a precursor was made by the process described in Example I using 228.3 g(1.0 M) of 4,4'-diamino diphenyl sulfone as the only diamine component.The resinoid was foamed by the method described in Example II.

The product was a yellow, rigid, brittle foam with a thick continuousskin on the outside and a rather nonuniform cell structure. When thefoam was subjected to the destructive test described in Example III, itcrumbled into a powder.

EXAMPLE V

In another test of the character described in the preceding example, anddesigned to show that heretofore suggested copolymers are no moresuitable for our purposes than the homopolymers, the procedure ofExample I was repeated using 54.07 g (0.5 M) of meta-phenylene diamineand 114.15 g (0.5 M) of 4,4'-diamino diphenyl sulfone as the diaminecomponent. The resinoid was foamed by the method of Example II.

When this foam was subjected to the destructive test of Example III, itwas completely destroyed.

EXAMPLE VI

To demonstrate that the diamine constituents of our novel polyimidefoams can be varied within the limits discussed above, the procedure ofExample I was repeated using 54.07 g (0.5 M) of meta-phenylene diamineand 54.6 g (0.5 M) of 2,6-diamino pyridine as the diamine component. Theresinoid was formed by the Example II method.

The product was a very resilient, low density foam which had ahomogeneous cell structure and a light color. This foam was subjected tothe destructive test of Example II. It remained resilient and maintainedits original cell structure.

The results of the foregoing and several additional tests are summarizedin Table 1 below. In each case the resinoid was prepared according tothe process of Example I, foamed according to the procedure of ExampleII, and tested as described in Example III. The ratio of diamines wasequimolar unless specified otherwise.

                                      TABLE 1                                     __________________________________________________________________________                               300° F. Destructive Test                    Example                                                                            Ester      Polyamines Effect on Foam                                     __________________________________________________________________________    I    diethyl ester of BTA                                                                     2,6 DAP, pDADPS                                                                          Resilient; no damage                               IV   diethyl ester of BTA                                                                     pDADPS     Reduced to a powder                                V    diethyl ester of BTA                                                                     mPDA, pDADPS                                                                             Foam destroyed                                     VI   diethyl ester of BTA                                                                     2,6 DAP, mPDA                                                                            Resilient, no damage                               VII  diethyl ester of BTA                                                                     mPDA       Foam destroyed                                     VIII diethyl ester of BTA                                                                     mDADPS     Reduced to a powder                                IX   diethyl ester of BTA                                                                     2,6 DAP, mDADPS                                                                          Resilient; no damage                               X    diethyl ester of BTA                                                                     2,5 DAP (.25M)                                                                           Resilient; no damage                                               mPDA (.75M)                                                   XI   diethyl ester of BTA                                                                     2,6 DAP (.1M)                                                                            Resilient; no damage                                               pDADPS (.15M)                                                                 mPDA (.75M)                                                   XII  diethyl ester of BTA                                                                     2,6 DAP    Carbonized powder residue                                                     after foaming step                                 XIII diethyl ester of BTA                                                                     mPDA, mDADPS                                                                             Reduced to a powder                                XIV  diethyl ester of BTA                                                                     pPDA, pDADPS                                                                             Reduced to a powder                                XV   diethyl ester of BTA                                                                     pPDA, mDADPS                                                                             Reduced to a powder                                XVI  diethyl ester of BTA                                                                     2,6 DAP, pDADPSu                                                                         Resilient; no damage                               XVII diethyl ester of BTA                                                                     2,6 DAP (.3M),                                                                           Resilient; no damage                                               mPDA (0.2M),                                                                  PDADPSu (0.5M)                                                __________________________________________________________________________     Legend:                                                                       BTA = 3,3',4,4'-benzophenonetetracarboxylic acid                              pPDA? = para-phenylene diamine                                                mPDA = meta-phenylene diamine                                                 2,6DAP = 2,5-diamino pyridine                                                 mDADPS = 3,3'-diamino diphenyl sulfone                                        pDADPS = 4,4'-diamino diphenyl sulfone                                        pDADPSu = 4,4'-diamino diphenyl sulfide                                  

Examples I, VI, IX, X, XVI, and XVII show that mixtures of aromaticdiamines and heterocyclic diamines such as 2,6-diamino pyridine makecopolyimide foams capable of withstanding severe mechanical stress at-300° F. without damage. Copolyimide foams made from such diamines arealso thermostable and unaffected by prolonged heating at temperatures ashigh as 600° F.

The homopolyimide foams (Examples IV, VII, and VIII) were completelycollapsed or destroyed by subjecting them to mechanical stress at thistemperature.

The copolyimide foams which did not possess a heterocyclic moiety in thepolymer chain (Examples V, XIII, XIV, and XV) fared no better. Thisshows that it is not enough merely to employ a mixture of diaminesindiscriminately selected from those of aromatic and heterocycliccharacter or entirely aromatic in character as taught by the above-citedAcle patents if a foam usable under stress at cryogenic temperatures isthe goal.

Example XII shows that a mixture of diamines is essential even if aheterocyclic diamine is employed. The attempt to convert a precursorcontaining only one diamine to a polyimide produced a carbonizedresidue, not a foam, even though a heterocyclic diamine was employed.

Examples XI and XVII show that more than two diamines can be employed.Ordinarily, however, any advantages gained by using more than twodiamines will not be significant.

The differences in the properties possessed by the various polyimidesare possibly attributable to the secondary processes which take placeduring the imidization reactions. These secondary processes producestructures having intermolecular bondings or cross links, and these maybe responsible for an increase in elasticity.

That structures of this character may be created is believed to be dueto the low basicity constant of 2,6'-diamino pyridine and otherheterocyclics with nitrogen in the ring, this resulting from theelectron-attracting property of the nitrogen atom.

It can logically be assumed that the imide ring formed with a lowbasicity diamine such as a diamino pyridine is in a state of continuouscleavage and reformation at polymerization temperatures. This results inthe formation of intermolecular imide links in addition to the expectedintramolecular imide rings. The products can be represented by thefollowing structural formula: ##STR1## where R' and R represent theheterocyclic and tetra acid moieties, respectively.

It is, again logically, assumed that the equilibrium between theintermolecular and intramolecular structures is determined by theconcentration of heterocyclic diamine in the original composition.Intermolecular condensation would be expected to prevail at highconcentrations of heterocyclic diamine and could be practicallyquantitative when only a heterocyclic diamine is present in theresinoid. In this case the final product would be a highly crosslinkedpolymer having no characteristic polyimide properties. This is confirmedby the test of Example XII where, as discussed above, the foaming stepproduced a carbonized residue rather than a polyimide foam.

The invention may be embodied in other specific forms without departingfrom the spirit or essential characteristics thereof. The presentembodiments are therefore to be considered in all respects asillustrative and not restrictive, the scope of the invention beingindicated by the appended claims rather than by the foregoingdescription; and all changes which come within the meaning and range ofequivalency of the claims are therefore intended to be embraced therein.

What is claimed and desired to be secured by Letters Patent is:
 1. Acopolyimide foam precursor which is a resinoid and which contains: oneor more lower alkyl esters of a benzophenonetetracarboxylic acid; two ormore diamines, at least one of said diamines being heterocyclic andhaving nitrogen in the ring, another of said diamines being a para- ormeta-substituted aromatic diamine which is free of aliphatic moieties,and any additional diamine being either a heterocyclic or an aromaticdiamine as aforesaid; and from 0.1 to 10 parts by weight of a siliconesurfactant per 100 parts of ester and diamine constituents.
 2. Acopolyimide foam precursor as defined in claim 1 in which heterocyclicand aromatic diamines are present in a ratio of from 5-95 parts byweight of heterocyclic diamine or diamines to 95-5 parts of aromaticdiamine or diamines.
 3. A copolyimide foam precursor as defined in claim1 in which the diamines are selected from the group consisting of3,3'-diamino diphenyl sulfone, 4,4'-diamino diphenyl sulfone,4,4'-diamino diphenyl sulfide, 3,3'-diamino diphenyl ether, 4,4'-diaminodiphenyl ether, benzidene, meta-phenylene diamine, para-phenylenediamine, 2,6-diamino pyridine, and 3,5-diamino pyridine.
 4. Acopolyimide foam precursor as defined in claim 1 in which any esterpresent in the precursor is a C₁ -C₃ ester of3,3',4,4'-benzophenonetetracarboxylic acid.
 5. The process of preparinga copolyimide foam precursor which includes the steps of: dissolving abenzophenonetetracarboxylic acid or anhydride in an esterifying agent oragents to form one or more lower alkyl esters of the acid; dissolvingtwo or more diamines in the product thus formed in an amount such thatthe imide forming functionalities are substantially equimolar, one ofsaid diamines being heterocyclic and having nitrogen in the ring,another of said diamines being a para- or meta-substituted aromaticdiamine which is free of aliphatic moieties, and any additional diaminebeing either a heterocyclic or an aromatic diamine as aforesaid;removing excess esterifying agent; and, after excess esterifying agenthas been removed, adding to the precursor from 0.1 to 10 parts by weightof a silicone surfactant based on the weight of the ester and diamineconstituents.
 6. A process as defined in claim 5 in which, following theaddition of the surfactant, the product is heated under reduced pressureto remove excess esterifying agent and is then comminuted.
 7. A processas defined in claim 6 in which the comminuted material is heated at atemperature in the range of 150°-250° F. until the volatiles content ofthe material is from one to five percent lower than the theoreticalvolatiles content of the ester and diamine constituents.
 8. A process asdefined in claim 7 in which the material is comminuted after heating itto reduce the volatiles content.
 9. A process as defined in claim 6 inwhich the comminuted material is heated at a temperature in the range of150°-250° F. until its melting point increases to a temperature in therange of 200°-225° F.
 10. A process as defined in claim 9 in which thematerial is thereafter comminuted.
 11. The process of preparing acopolyimide foam precursor which includes the steps of: dissolving atetracarboxylic acid or anhydride in an esterifying agent or agents toform one or more alkyl esters of the acid; dissolving two or morediamines in the product thus formed in an amount such that the imideforming functionalities are substantially equimolar, one of saiddiamines being heterocyclic and having nitrogen in the ring, another ofsaid diamines being a para- or meta-substituted aromatic diamine whichis free of aliphatic moieties, and any additional diamine being either aheterocyclic or an aromatic diamine as aforesaid; removing excessesterifying agent; and heating the resulting material at a temperaturein the range of 150° to 250° F. until its volatiles content is from oneto five percent lower than the theoretical volatiles content of theester and diamine constituents.
 12. A copolyimide foam precursor whichis a resinoid obtained by combining one or more esters of atetracarboxylic acid with two or more diamines, one of said diaminesbeing heterocyclic and having nitrogen in the ring, another of saiddiamines being a para- or meta-substituted aromatic diamine which isfree of aliphatic moieties, any additional diamine being either aheterocyclic or an aromatic diamine as aforesaid, and the volatilescontent of the resinoid being from one to five percent lower than thetheoretical volatiles content of the ester or esters and the diamines.13. A process as defined in claim 5 in which heterocyclic and aromaticdiamines are present in the precursor in a ratio of from 5-95 parts byweight of heterocyclic diamine or diamines to 95-5 parts of aromaticdiamine or diamines.
 14. A process as defined in claim 5 in which thediamines in the precursor are selected from the group consisting of3,3'-diamino diphenyl sulfone, 4,4'-diamino diphenyl sulfone,4,4'-diamino diphenyl sulfide, 3,3'-diamino diphenyl ether, 4,4'-diaminodiphenyl ether, benzidene, meta-phenylene diamine, para-phenylenediamine, 2,6-diamino pyridine, and 3,5-diamino pyridine.
 15. A processas defined in claim 5 in which any ester present in the precursor is aC₁ -C₃ ester of 3,3',4,4'-benzophenonetetracarboxylic acid.
 16. Aprocess as defined in claim 11 in which heterocyclic and aromaticdiamines are present in the precursor in a ratio of from 5-95 parts byweight of heterocyclic diamine or diamines to 95-5 parts of aromaticdiamine or diamines.
 17. A process as defined in claim 11 in which thediamines in the precursor are selected from the group consisting of3,3'-diamino diphenyl sulfone, 4,4'-diamino diphenyl sulfone,4,4'-diamino diphenyl sulfide, 3,3'-diamino diphenyl ether, 4,4'-diaminodiphenyl ether, benzidene, meta-phenylene diamine, para-phenylenediamine, 2,6-diamino pyridine, and 3,5-diamino pyridine.
 18. A processas defined in claim 11 in which any ester present in the precursor is aC₁ -C₃ ester of 3,3', 4,4'-benzophenonetetracarboxylic acid.
 19. Acopolyimide foam precursor as defined in claim 12 in which heterocyclicand aromatic diamines are present in a ratio of from 5-95 parts byweight of heterocyclic diamine or diamines to 95-5 parts of aromaticdiamine or diamines.
 20. A copolyimide foam precursor as defined inclaim 12 in which the diamines are selected from the group consisting of3,3'-diamino diphenyl sulfone, 4,4'-diamino diphenyl sulfone,4,4'-diamino diphenyl sulfide, 3,3'-diamino diphenyl ether, 4,4'-diaminodiphenyl ether, benzidene, meta-phenylene diamine, para-phenylenediamine, 2,6-diamino pyridine, and 3,5-diamino pyridine.
 21. Acopolyimide foam precursor as defined in claim 12 in which any esterpresent in the precursor is a C₁ -C₃ ester of3,3',4,4'-benzophenonetetracarboxylic acid.
 22. A copolyimide foamprecursor as defined in claim 12, in which all ester constituents arelower alkyl esters of a benzophenonetetracarboxylic acid, said resinoidalso containing from 0.1 to 10 parts by weight of a silicone surfactantper 100 parts of ester and diamine constituents.
 23. A copolyimide foamprecursor which is a resinoid obtained by combining one or more estersof a tetracarboxylic acid with two or more diamines: all esterconstituents being lower alkyl esters of a benzophenonetetracarboxylicacid; one of said diamines being heterocyclic and having nitrogen in thering, another of said diamines being a para- or meta-substituted diaminewhich is free of aliphatic moieties, and any additional diamine beingeither a heterocyclic or an aromatic diamine as aforesaid; the meltingpoint of said precursor being in the range of 200°-225° F.; and saidresinoid also containing from 0.1 to 10 parts by weight of a siliconesurfactant per 100 parts of ester and diamine constituents.